[volt-nuts] Mechanical choppers for precision instruments
Randall White
randall.m.white at gmail.com
Sat Mar 20 16:10:46 UTC 2010
Hi Frank,
Do you have any additional information about this? I have a Keithley 148
nanovoltmeter with a now-defunct chopper, and I had considered trying to
either rebuild it, or replace it with a low thermal-EMF reed relay. I
hadn't thought of using a FET. Do you happen to know which Fluke designs
implemented this?
The thermal EMF can probably be dealt with by appropriate packaging and
mounting. The clock feedthrough / charge injection may require a little
more thought. The existing chopper in the 148 has a bucking coil that
can be adjusted to null out the coupling to the input line from the
relay coil.
-Randall
> Subject:
> [volt-nuts] Fluke 883AB differential voltmeter
> From:
> "Dr. Frank Stellmach" <drfrank.stellmach at freenet.de>
> Date:
> Fri, 19 Mar 2010 22:57:12 +0100
> To:
> volt-nuts at febo.com
>
> To:
> volt-nuts at febo.com
>
>
>>> ..Final problem was it would not zero properly.
> The zero circuit uses two diodes to make a +/- 0.6V supply driving the
> offset pot.
> The unit was offset so that you could not reach zero and the offset
> pot had fairly little range.
> Adjusting the chopper didn't help.
> I replaced the diodes with three 1.2V reference chips (making a -1.2
> to +2.4V supply).
> This centered the zero adjustment and gave better range...
>
>
> Hi Mark,
>
> this problem seems to be related to the mechanical, reed contact chopper.
>
> This device had to be replaced very often, when I worked in a cal lab
> about 30 years ago..
>
> I wonder if the unit realy works oK after your modification.
>
> Try to replace the mechanical contact by a FET., like in later Fluke
> designs..
>
> Frank
>
>
>
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